Thus an abstractive set
is effectively the entity meant when we consider an instant of time
without temporal extension. It subserves all the necessary purposes of
giving a definite meaning to the concept of the properties of nature at
an instant. I fully agree that this concept is fundamental in the
expression of physical science. The difficulty is to express our
meaning in terms of the immediate deliverances of sense-awareness, and I
offer the above explanation as a complete solution of the problem.

In this explanation a moment is the set of natural properties reached by
a route of approximation. An abstractive series is a route of
approximation. There are different routes of approximation to the same
limiting set of the properties of nature. In other words there are
different abstractive sets which are to be regarded as routes of
approximation to the same moment. Accordingly there is a certain amount
of technical detail necessary in explaining the relations of such
abstractive sets with the same convergence and in guarding against
possible exceptional cases. Such details are not suitable for exposition
in these lectures, and I have dealt with them fully elsewhere[5].

[5] Cf. _An Enquiry concerning the Principles of Natural Knowledge_,
Cambridge University Press, 1919.

It is more convenient for technical purposes to look on a moment as
being the class of all abstractive sets of durations with the same
convergence. With this definition (provided that we can successfully
explain what we mean by the 'same convergence' apart from a detailed
knowledge of the set of natural properties arrived at by approximation)
a moment is merely a class of sets of durations whose relations of
extension in respect to each other have certain definite peculiarities.
We may term these connexions of the component durations the 'extrinsic'
properties of a moment; the 'intrinsic' properties of the moment are the
properties of nature arrived at as a limit as we proceed along any one
of its abstractive sets. These are the properties of nature 'at that
moment,' or 'at that instant.'

The durations which enter into the composition of a moment all belong to
one family. Thus there is one family of moments corresponding to one
family of durations. Also if we take two moments of the same family,
among the durations which enter into the composition of one moment the
smaller durations are completely separated from the smaller durations
which enter